This invention relates to a method for fabricating conductors in very large scale integrated (VLSI) circuits and to conductors obtained using the method.
It is known that electromigration and hillock formation within aluminum conductors can seriously reduce VLSI device yield since these features cause device failures. Electromigration is caused by aluminum atom transfer at the grain boundary or at surface regions of the aluminum conductor due to high density electron flow. It can result in open circuit. Hillock growth is caused by the difference in thermal expansion coefficients between the aluminum and underlying silicon dioxide. It is known to add copper to the aluminum to improve electromigration resistance and reduce hillock growth. However, aluminum-copper films are difficult to plasma etch because copper compounds have low vapour pressure. Also hillocks are still not completely eliminated.
Recently it has been found that aluminum-silicon coated by a refractory metal or refractory metal silicide layer shows an appreciable improvement in electromigration resistance and suppression of hillock growth (D. S. Gardner et al, 1984, Proceedings of IEEE, VLSI Multilevel Interconnection Conference, June 21-22, New Orleans, page 68 and Shima et al, Ibid, page 61). However, the resistivity of this composite conductor is increased and its dry etching is difficult because of the existence of a native aluminum oxide at the interface between the aluminum-silicon alloy and the refractory metal or refractory metal silicide layer.